Chemical vapor deposition has a range of equipment reactor designs, with each producing slightly different types of film quality. Chemical vapor deposition reactors are broadly categorized based upon the reaction chamber pressure regime used during the operations: Atmospheric-pressure chemical vapor deposition (APCVD) reactors and reduced-pressure reactors. The reduced-pressure chemical vapor deposition reactors are of two general types. First, there is low-pressure chemical vapor deposition (LPCVD) reactors where the energy input is thermal. Second, there are plasma-assisted chemical vapor deposition reactors, either plasma enhanced chemical vapor deposition (PECVD) or high-density plasma chemical vapor deposition (HDPCVD) where the energy is partially supplied by a plasma as well as thermally.
APCVD generally operates in a mass-transport limited regime. At any given time, there may not be sufficient gas molecules present at the wafer surface for a reaction to occur. Therefore, the reactor must be designed to have optimum reactor gas flow to every wafer in the system. The most common use of APCVD is the deposition of silicon dioxide and doped oxides. These films have been used traditionally as an interlayer dielectric, as a protective overcoat, or to planarize a non-uniform surface. APCVD can be utilized to deposit silicon dioxide with silane typically at a low temperature of about 450 to 500° C. Silicon dioxide can also be deposited with TEOS-ozone at a temperature about 400° C.
LPCVD operates at a minimum vacuum of about 4.1 to 5 torr at temperatures ranging between 300 and 900° C. LPCVD reactors typically operate in the reaction-rate limited regime. In this reduced-pressure regime, the diffusivity of the reacting gas molecules increases so that the mass-transfer of the gas to the wafer no longer limits the rate of the reaction. Because of this transfer state, the gas-flow conditions inside the reactor are not important, permitting the reactor design to be optimized for high wafer capacity, for example, wherein wafers can be closely spaced. Films are uniformly deposited on a large number of wafer surfaces as long as the temperature is tightly controlled. LPCVD reactor designs favor a hot-wall reactor type so that the uniform temperature control is achievable of all over a large operating length.
Three major types of chemical vapor deposition equipment rely on plasma energy, in addition to thermal energy to initiate and sustain the chemical reaction necessary for chemical vapor deposition. The plasma-assisted chemical vapor deposition reaction necessary to form a film occurs with RF power is use to breakup gas molecules in a vacuum. The frequency of the RF power depends on the application, with a typical frequencies found at about 40 kHz, 400 kHz, 13.56 MHz and 2.45 GHz (microwave power). The molecular fragments or radicals are chemically reactive species and readily bond to other atoms to form a film at the wafer surface. Gaseous byproducts are removed by a vacuum pumping system. The wafer is heated in order to assist the surface reactions and reduced the level of undesirable contaminants such as hydrogen.
Plasma-enhanced chemical vapor deposition (PECVD) uses plasma energy to create and sustain the chemical vapor deposition reaction. An important difference between PECVD and LPCVD is the much lower PECVD deposition temperature. For instance, silicon nitride is deposited using LPCVD at 800 to 900° C. On the other hand, silicon nitride is deposited using PECVD at a temperature of about 350° C. The PECVD reactor typically is a cold-wall plasma reactor with the wafer heated in its chuck while the remaining parts of the reactor are unheated. Deposition parameters must be controlled to ensure the temperature gradient does not affect the film thickness uniformity. Cold-wall reactors create fewer particles and require a less downtime for cleaning.
High-density plasma chemical vapor deposition (HDPCVD) is a high-density mixture of gases at low pressure that is directed towards the wafer surface in a reaction chamber. The main benefit of HDPCVD is that it can deposit films to fill high aspect ratios with a deposition temperature range of 300-400° C. HDPCVD was initially developed for interlayer dielectric applications, but it has also been used for deposition of ILD-1, shallow trench isolation, etch-stop layers, and deposition of low-k dielectrics. The HDPCVD reaction involves a chemical reaction between two or more gas precursors. To form the high-density plasma, a source excites the gas mixture with RF or microwave power and directs the plasma ions into a dense region above the wafer surface. There are different high-density plasma sources, such as electron cyclotron resonance, inductively coupled plasma, and Helicon. Much of the challenge for the use of high-density plasma is related not only to the performance of the plasma source but also to the details of the chamber designs so that the technology works in high-volume wafer fabrication. A particular problem is that the high-density plasma will increase the thermal load to the wafer. This condition leads to high wafer temperatures. As a result, HDPCVD reactors typically include a cooling system for the wafer, which typically includes applying a backside blanket of helium gas to the wafer through access ports in the electrostatic chuck. This action creates a thermally conductive path between the wafer and the electrostatic chuck, thus cooling the wafer and the chuck.
The HDPCVD oxide process usually uses silane as the silicon precursor and oxygen as the oxygen precursor. Argon is added in the process to enhance the sputtering etch effect. HDPCVD can also deposit PSG and FSG by reacting with phosphorus and silicon tetrafluoride.
Problems in this sputtering are continued to challenge those skilled in the art. FIG. 1 illustrates a high sputter rate process in which molecules 10 such as silicon dioxide are sputtered into a trench 12 to fill the trench and form a silicon dioxide layer 14. As illustrated in FIG. 1, in a high sputter rate process, the silicon dioxide molecules 10 often bounce off of the corner defining the trench 12 and tend to stick to the sidewalls 16 defined in the trench 12. As a result, as illustrated in FIG. 2, a birds peak 18 tends to develop along the sidewall 16 of the trench. Eventually, sidewall birds peaks 18 converge on each other creating a void 20 in the trench as illustrated in FIG. 3.
FIG. 4 illustrates a relatively low sputter rate process in which molecules 10, such as silicon dioxide are sputtered into a trench 12. However, in this process because the sputter rate is relatively low, the molecules 10 do not tend to stick to the sidewalls 16 of the trench, but tend to fall towards the bottom 22 of the trench 12. However, even using a relatively low sputter rate process, it's difficult to control the process without producing voids in the trench. FIG. 5 illustrates a void 20 formed in the trench when the deposition to sputter ratio is about 10 for silicon dioxide being deposited in the trench. FIG. 6 illustrates a void 20 that develops when the deposition to sputter ratio is increased to about 20 for silicon dioxide being deposited in the trench.
The present invention provides alternatives to the prior art.